Annular magnet head of the kind comprising a solid core for magnetic recording and reproduction



Jan. 20, 1959 w. K. WESTMIJZE 2,870,266

ANNULAR MAGNET HEAD OF THE KIND COMPRISING A soup com: FOR MAGNETIC RECORDING AND REPRODUCTION Filed 001;. 29, 195:5

V J E GAPHQGHT ,5

lN-mea. Cane I CUMFEQEucE C INVENTOR WILLEM KLAAS WESTMIJZE AGENT ANNULAR MAGNET HEAD OF THE KIND COM- PRISING A SOLID CORE FOR MAGNETIC RE- CORDING AND RETRODUCTION Willem Kiaas Westmijze, Eindhoven, Netherlands, as-

signor, by mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application October 29, 1953, Serial No. 389,044

Claims priority, application Netherlands November 11, 1952 3 Claims. (Cl. 179-1002) The invention relates to a magnet head for magnetic recording or reproduction, comprising a core constituted by one or more solid bodies having at least one gap.

The term annular magnet head is to be understood to mean herein not only a magnet head comprising a round core, but also constructions formed by straight parts.

In the construction of such magnet heads the general tendency is to minimize the electrical losses in the circuit. It appears for example, in the reproduction process, that the flux emanating from the magnetic record passing by, may pass via two magnetic parts from one side of the gap along which the magnetic record medium moves to the other side. That is, the flux may pass, on the one hand, through the gap itself, comprising air or another non-magnetic substance, and on the other hand, through the material of the core surrounded by the coil of the head. These two courses will be termed hereinafter the air circuit and the rear circuit, respectively. It will be obvious in this respect that the output of the magnet head will be the higher, the greater the part of the total flux passing through the rear circuit and hence through the coil. It follows therefrom that the output varies with the ratio between the reluctances of the two said paths.

It appears moreover that at the occurrence of eddy current losses the reluctance of the rear circuit becomes complex and increases strongly its absolute value. Thus, at constantly increasing eddy current losses, the reluctance of the rear circuit becomes comparable with that of the air circuit, the output thus decreasing gradually.

In the recording process, the occurrence of inductance currents in the core will reduce the output, since the reluctances of the two said paths become comparable.

In the construction of the aforesaid magnet heads, efforts have therefore been constantly made to minimize the electrical losses in the circuit. It is known to build up the core of the magnet head concerned for this purpose from lamellae of the kind used for example in the manufacture of transformers, chokes and the like. The lamellae are capable, due to their relative insulation, of keeping the eddy currents below a definite value within a given frequency range. With respect to the solid core the lamination has, of course, the disadvantage of the comparatively high cost of manufacture involved and the greater difficulty in working and mounting. This disadvantage is, in certain cases, considered to be serious enough to develop heads comprising a solid core of electrically non-conductive, magnetic material, for example, ferroxcube. In this case, the nature of the substance constitutes a physical obstacle to the eddy currents. This type of head has a limitation in that the material must be sintered, so that it frequently cannot be shaped immediately in the desired form and an aftertreatment is required, which is carried out with difficulty due to the brittleness of the material.

The invention has for its object to provide a magnet retested Jan. 20, 1959 6 db, preferably not more than 4 db.

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In accordance with the present invention, the magnet head has dimensions which are such that, despite the increase in reluctance of the rear circuit due to eddy current losses for the magnetic material of the core concerned, the reluctance of the rear circuit is at the most equal to, and preferably lower than, that of the gap along which the magnetic record medium moves, even at the maximum frequency to be recorded or reproduced. It is thus possible to use electrically conductive materials with the aforesaid advantages. The material can be worked and finished more easily and the core may be cast; a method which is not carried out with electrically non-conductive, magnetic materials.

With respect to proportioning, it has been found that it is desirable that the product of the height of the gap 11 and the inner circumference of the core 0 should be lower than a constant equal to be larger than ten times the height h of the gap, along which the magnetic record medium is moved. In the r formula, w designates the width of the gap, n is a physical constant equal to the permeability of vacuum in the unit system used, p. is the permeability and r is the specific resistance of the core material, while wmax, designates the maximum frequency to be recorded or reproduced.

Where reference is made to a height of the gap of approximately 0.1 mm., it is advantageous to use, as a magnetic material for the core, a material having a high resistance to wear.

On the basis of the aforesaid discussion, those skilled in the art will act as follows in designing a magnet head according to the invention. With a view to the magnetic record medium or tape material to be used, the speed of the tape will first be determined. The width of the desired frequency range is chosen, so that the value w of the aforesaid constant for the product of h; and c is fixed. From the chosen value for w and the speed of the tape, the required width w of the gap can be derived. Then a choice is made from the available core materials and a material is chosen which preferably has a high resistance to wear, and, on the basis of the relationships referred to above, h; and c are fixed. By experience it has been found to be necessary to choose a value for the inner circumference c of the core which corresponds to that used hitherto in common practice. The value of the gap height I1 then found may not fulfill the condition that h should exceed 0.1 mm., so that for mechanical reasons this proportioning is to be rejected. In this case, a different material must be chosen, which does yield the aforesaid result. It has, for example, been found that the material known as Sendust is quite satisfactory, also because it has a high resistance to wear. Sendust has a composition of 9.7 percent silicon, 5.7 percent aluminum and the balance of iron. As an alternative, the first-mentioned material may be used, if the value of m is chosen to be such that all conditions, also that for h can be fulfilled. In this case, it is consequently to be considered as satisfactory to have a limited frequency range for the recording or the reproduction.

In order'that the invention may be readily carried into effect, it will now be described with reference to the accompanying drawing, wherein:

Fig. l is a lateral view of an embodiment of a magnet head according to the invention; and

Fig. 2 is a graphic presentation of a few characteristic curves relating to the proportioning of the core 2 of the embodiment of Fig. 1.

Referring to Fig. l, the magnetic record medium 1 is moved along a magnet head for reproducing magnetic records, the head comprising a solid core 2. The core 2, which is interrupted by the gap 4, is surrounded by the coil 3. The head is proportioned in a manner such that with the use of the magnetic core material concerned,

despite the increase in reluctance of the rear circuit due to eddy current losses, the reluctance of the rear circuit is at the most equal to and preferably smaller than that of the gap 4, even for the maximum frequency to be reproduced. In Fig. l, c designates the inner circumference of the core, w the width of the gap and h the height of the gap. The magnet head is proportioned to be such that the product of h, and c is smaller than a constant equal to w /&: I O mnx whereas, for practical reasons, h exceeds 0.1 mm. and the inner circumference c of the core exceeds ten times the height h, of the gap 4. Thus the frequency characteristic curve of the magnet head will drop by not more than 6, preferably not more than 4, db up to the highest frequencies to be reproduced.

Fig. 2 is a graphic presentation of the structural conditions in a few characteristic curves, wherein h is plotted as a function of the inner circumference c of the core in a manner corresponding to the aforesaid conditions.

Characteristic curves which may be utilized in designing the magnetic head of the present invention are labeled x, y and z. Curve x represents an exponential variation of gap height with inner core circumference; curve y represents a variation of inner core circumference at constant gap height; and curve 2: represents a linear variation of gap height with core circumference.

If a real range 5 is closed between the three characteristic curves x, y and z, the core may be made for the maximum frequency chosen for the electrically conductive material concerned, without incurring a drop of f more than 6 db of the frequency characteristic curve below the maximum frequency. It will be obvious that any material suitable for this purpose may be used for the manufacture of a magnet head according to the invention. Use may, for example, be made of ,u-metal.

It will otherwise be advantageous to choose from the said materials those which have a high resistance to wear; for example, Sendust or silicon iron, which favors the lifetime of the magnet head.

While the invention has been described by means of a specific example and in a specific embodiment, I do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. An annular magnetic head for recording on or reproducing from a magnetic record, signals having a given maximum frequency, said head comprising a solid core of magnetic material having an inner circumference and defining a non-magnetic gap, the product of the height of said gap and the length of said inner circumference being less than a constant equal to wherein w designates the width of the gap,-,u a physical constant equal to the permeability of vacuum in the unit system used, the permeability and r the specific resistance of the material of said core, and co said maximum frequency, said height having a value exceeding 0.1 mm. and said length having a value exceeding ten times the value of said height, whereby the reluctance of the core material is lower than the reluctance of said gap even at said maximum frequency.

2. An annular magnetic head as set forth in claim 1, wherein the material of said core comprises a material having a high resistance to wear.

3. An annular magnetic head as claimed in claim 1, wherein the material of said core comprises an iron silicon alloy having a high resistance to wear.

References Cited in the tile of this patent UNITED STATES PATENTS 2,469,444 Roys May 10, 1949 2,632,816 Gratain Mar. 24, 1953 2,653,189 Camras Sept. 22, 1953 2,715,659 Ibuka et al Aug. 16, 1955 

